1. Field of the Invention
This invention relates to the field of current integration circuits.
2. Description of the Related Art
It is often necessary to know the magnitude of a particular current over time. This can be determined with a current integrator.
Current integrators are well-known; a basic implementation is shown in FIG. 1. An operational amplifier A1 receives a current to be integrated Iin at its inverting input, with its non-inverting input grounded. A fixed integration capacitor C is connected between the op amp's output and inverting input. A switch SR is connected across capacitor C, which resets the integrator when closed. Input current Iin is integrated on capacitor C to produce an output voltage Vout from A1.
This arrangement suffers a number of shortcomings, however. If Vmax is the maximum output voltage that A1 can produce, then the maximum charge Qmax that can be stored on integration capacitor C without causing A1's output to become saturated is given by Qmax=Vmax*C. The total charge to be integrated is given by       Q    total    =            ∫      0              T        int              ⁢                  I        in            ⁢                          ⁢              ⅆ        t            (where Tint is the integration period), or Qtotal=Iin×Tint if Iin is constant. Capacitor C needs to store Qtotal, SO Qmax≧Qtotal, or C≧Qtotal/Vmax. Thus, to achieve a high Qtotal requires a large C value to ensure that the amplifier does not saturate. The area used by C can dominate the area of an integrated circuit die.
Another shortcoming of the circuit in FIG. 1 is resolution. If Vout were to be sampled by an n-bit analog-to-digital (A/D) converter, the resolution of the integrated charge would be limited to Qmax/2n. One approach to improving the resolution is shown in FIG. 2. An array of integration capacitors such as Ca, Cb and Cc are used to allow different integration gains to be selected, using respective switches Sa, Sb and Sc. However, this arrangement typically requires that the capacitors, and thus the integration gain, be selected before the input current is integrated. If the magnitude of the input current or charge is unknown, it is difficult to select the correct capacitance to provide an integration gain which maximizes the integrator's signal-to-noise ratio. Furthermore, the total integration capacitance needs to be chosen to accommodate the maximum anticipated input charge; i.e., Ca+Cb+Cc≧Qtotal/Vmax. Thus, this design also requires a large die area if a large input charge is to be accommodated.